We shall determine whether the slowdown of cell multiplication in Mg2 ion-deprived 3T3 cell cultures occurs in the G-1 period of the cell cycle or elsewhere, using autoradiographic and flow cytometric techniques. The results will be correlated with changes in the intracellular and surface bound concentrations of the 4 major cations. Similar studies will be done with a spontaneously transformed variant of 3T3 cells which requires much less Mg2 ion for maximal growth. Since the transformed cells also assume normal morphology in low Mg2 ion, other properties of the cells in low Mg2 ion such as anchorage independent growth will be studied. Divalent cation ionophores will be used to make rapid changes in concentration of intracellular Mg2 ion and Ca2 ion and the effects of these changes on uridine phosphorylation will be determined. The aim here is to evaluate rigorously the role of divalent cations in a relatively simple early response of cells to external stimuli. This will require detailed kinetic comparisons of changes in intracellular ions, uridine uptake, ion effects on uridine phosphorylation in cell extracts and a search for other possible effectors of the reaction.